Automotive safety systems have become increasingly common in the past few years, and, in many cases, their inclusion in automobiles are mandated by government and industry standards. Such electronic safety systems are typically equipped with an energy reserve. In case of power loss, these energy reserve systems supply enough power for the reliable operation of emergency automotive systems, for example, airbag and antilock brake systems. By using such an energy reserve, an airbag can still be deployed during a serious accident if a main battery connection is severed. These energy reserves typically consist of capacitors. During normal operation of the automobile, these capacitors are continually charged by the main car power supply and/or battery.
In order to ensure reliable and safe operation of these energy reserve systems, however, the functionality of these systems are frequently tested by performing an energy reserve measurement (ERM). More particularly, the capacity of the energy reserve as well as the reliability of its connection to the system is verified. If the energy reserve fails or does not function properly because of an electrical failure or loose electrical connection, corrective actions, such as alerting the driver with a warning light, disabling certain functions, or even setting the device in a fail-safe mode, can be taken. Lately, the inclusion of reliable ERM systems have become more critical as the automotive industry is moving toward safety requirements compliance with such standards as IEC 61508, which defines various safety integrity levels (SIL).
One conventional method used to test an energy reserve system is to disconnect the energy reserve storage capacitor from its power supply and measure its electrical characteristics. These measurements are conventionally made while the automobile is starting up, for example, when the key is being turned in the ignition. One drawback of this method of testing is that the ERM is being made only while the car is starting up, therefore, a component failure that occurs after the ERM, or intermittent failures that only occur while the car is operating will not be detected. An example of this is when a capacitor with a bad solder joint that loses connectivity due to vibration. Another possible drawback is a lack of connectivity due to a failed ERM test cycle. For example, the test circuit that disconnects the energy reserve storage capacitor from the power supply during testing suffers a failure and does not reconnect the capacitor when the test cycle is complete.
What are needed are reliable systems and methods for energy reserve measurement (ERM) for electronic automotive safety systems.